(1) Field of the Invention
This invention relates to radiation detecting apparatus of the direct conversion type used in the medical, industrial, nuclear and other fields, and more particularly to a technique of suppressing creeping discharges due to a bias voltage applied to a radiation sensitive semiconductor layer.
(2) Description of the Related Art
Radiation (e.g. X rays) detecting apparatus include the indirect conversion type which first converts radiation (e.g. X rays) into light and then converts the light into electric signals by photoelectric conversion, and the direct conversion type which converts incident radiation directly into electric signals with a radiation sensitive semiconductor layer. The latter, direct conversion type apparatus has a voltage application electrode formed on a front surface of the radiation sensitive semiconductor layer to which a predetermined bias voltage is applied, and carrier collection electrodes formed on a back surface of the radiation sensitive semiconductor layer for collecting carriers generated by incident radiation. The carriers are taken out as radiation detection signals, thereby enabling a detection of the radiation.
Certain of the conventional radiation detecting apparatus of the direct conversion type employ a thick layer of an amorphous semiconductor such as amorphous selenium as the radiation sensitive semiconductor layer. An amorphous semiconductor may simply be processed to form a thick and wide layer by vacuum deposition or the like, and therefore is suited for a two-dimensional array structure requiring a large, thick layer.
As shown in FIG. 1, a conventional two-dimensional array type radiation detecting apparatus includes a thick amorphous semiconductor layer 1 for generating electron-hole pairs (carriers) in response to incident radiation, a voltage application electrode 2 formed on the front surface of semiconductor layer 1 to which a bias voltage is applied, and a plurality of carrier collection electrodes 3 arranged in a two-dimensional matrix form on the back surface of semiconductor layer 1. Each carrier collection electrode 3 has, connected thereto, a charge storing capacitor Ca and a charge reading switching element (e.g. a thin film transistor) 4 which is normally turned off. Charges accumulating in the capacitors Ca as a result of incident radiation are read as radiation detection signals through the switching elements 4 turned on.
The radiation detecting apparatus having the two-dimensional array structure shown in FIG. 1 may be used in a fluoroscopic apparatus for detecting transmitted X-ray images. In this case, fluoroscopic images are acquired based on the radiation detection signals outputted from the radiation detecting apparatus.
However, the conventional radiation detecting apparatus noted above has a drawback that the bias voltage applied to the thick amorphous semiconductor layer 1 tends to result in creeping discharges. The creeping discharges are caused by dielectric breakdowns occurring along surfaces from edges 2a of the voltage application electrode 2 to edges 1a of the thick amorphous semiconductor layer 1, before being grounded, as shown in FIG. 1.
In the case of a fluoroscopic image, for example, the creeping discharges result in noise in the radiation detection signals to become a detriment to image quality. The creeping discharges may be suppressed by reducing the bias voltage. However, an amorphous semiconductor is inferior to a single crystal semiconductor in carrier transport characteristics, and cannot demonstrate sufficient detecting sensitivity with a low bias voltage.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiation detecting apparatus which suppresses creeping discharges due to a bias voltage applied to a radiation sensitive semiconductor layer.
The above object is fulfilled, according to this invention, by a radiation detecting apparatus having a radiation sensitive semiconductor layer for generating carriers, i.e. electron-hole pairs, in response to incident radiation, a voltage application electrode formed on a front surface of the semiconductor layer for receiving a bias voltage applied thereto, carrier collection electrodes formed on a back surface of the semiconductor layer, and charge storing capacitors and charge reading switching elements connected to the carrier collection electrodes, the switching elements being normally turned off, charges accumulating in the capacitors as a result of the incident radiation being read as radiation detection signals through the switching elements turned on, wherein:
the radiation sensitive semiconductor layer is a thick amorphous semiconductor layer;
a carrier selective high resistance film is formed between the thick amorphous semiconductor layer and the voltage application electrode to entirely cover a surface of the thick amorphous semiconductor layer; and
an electrodeless region extends throughout a circumference of the thick amorphous semiconductor layer between edges of the voltage application electrode and edges of the thick amorphous semiconductor layer.
With the apparatus according to this invention, radiation to be detected is emitted while a bias voltage is applied to the voltage application electrode formed on the front surface of the radiation sensitive amorphous semiconductor layer. Then, charges accumulate in the charge storing capacitors connected to the carrier collection electrodes, in a quantity corresponding to the carriers generated by incident radiation in the amorphous semiconductor layer. As the charge reading switching elements are turned on, the charges having accumulated are read as radiation detection signals through the switching elements.
Thus, when detecting radiation, the carrier selectivity of the carrier selective high resistance film blocks an injection of those carriers (electrons or holes) not contributing to the detection of radiation but becoming dark currents, thereby to suppress dark currents. An injection of the carriers that contribute to the detection of radiation is not blocked, thereby to maintain signal response characteristics.
The surface of the amorphous semiconductor layer is completely covered by the carrier selective high resistance film. This construction prevents crystallization due to moisture and the like of the amorphous semiconductor layer to avoid lowering of the surface resistance. In addition, the electrodeless region is formed throughout the circumference between edges of the voltage application electrode and edges of the amorphous semiconductor layer. The voltage application electrode is surrounded by the carrier selective high resistance film having a high surface resistance. Since sufficient surface voltage endurance is provided between the voltage application electrode and the grounding side, creeping discharges from the radiation sensitive amorphous semiconductor layer due to the bias voltage are suppressed. Consequently, sufficient detection sensitivity is secured by applying a high bias voltage.
In the apparatus according to this invention, the carrier collection electrodes may be formed in a large number and arranged in a two-dimensional matrix, each of the carrier collection electrodes having one each of the charge storing capacitors and the charge reading switching elements to constitute a two-dimensional array structure. Then, each radiation detecting unit is capable of locally detecting radiation, to enable measurement of a two-dimensional distribution of radiation intensity.
Where the carrier selective high resistance film is a p-type conducting film, a negative bias voltage is applied to the voltage application electrode. This prevents an injection of electrons that do not contribute to the detection of radiation but become dark currents. An injection of holes that contribute to the detection of radiation is allowed, thereby to detect radiation reliably.
Where the carrier selective high resistance film is an n-type conducting film, a positive bias voltage and the n-type carrier selective high resistance film prevent an injection of holes that do not contribute to the detection of radiation but become dark currents, and allows an injection of electrons that contribute to the detection of radiation. Thus, radiation may be detected reliably.
Further, the electrodeless region, preferably, has a width in a range of B mm to 3 B mm, B being a numerical value when an absolute value of the bias voltage is expressed in kV (kilovolt). With the width of the electrodeless region set to the above preferred range, sufficient surface voltage endurance is secured for inhibiting creeping discharges due to the bias current reliably. The above range does not substantially diminish the sensitive area (i.e. size of the detecting area) of the amorphous semiconductor layer. Thus, the creeping discharges due to the bias voltage are suppressed, and at the same time full use is made of the amorphous semiconductor layer suited for realizing an enlarged area.
In the apparatus according to this invention, the thick amorphous semiconductor layer, preferably, has a thickness in a range of 0.5 mm to 1 mm. With the thickness of the amorphous semiconductor layer set to the above preferred range, radiation is absorbed sufficiently by the semiconductor layer without passing therethrough. Preferably, the thick amorphous semiconductor layer is formed of amorphous selenium (a-Se). The thick amorphous selenium layer is particularly suited for realizing an enlarged detection area.
The carrier selective high resistance film, preferably, has a surface resistance of at least 108xcexa9/xe2x96xa1. With the surface resistance of the carrier selective high resistance film set to the above preferred value, creeping discharges are suppressed by the high surface resistance.
The carrier selective high resistance film, preferably, has a thickness in a range of 0.01 xcexcm to 10 xcexcm. With the thickness of the carrier selective high resistance film set to the above preferred range, an injection of unwanted carriers is inhibited while sufficiently allowing an injection of required carriers.